1. Field of the Invention
The present invention relates to an image display apparatus, particularly a face- or head-mounted type image display apparatus comprising a right-eye image display device for displaying a right-eye image which is exclusively seen by a right-eye of a user and a left-eye image display device for displaying a left-eye image which is exclusively seen by a left eye of the user.
2. Description of Related Art
In Japanese Patent Application Laid-open Publication Kokai Hei 4-61495, there is described an image display apparatus of a kind mentioned in the preamble. FIG. 1 is a block diagram showing this known image display apparatus. The image display apparatus comprises right-eye image displaying circuit 1R, left-eye image displaying circuit 1L and switching circuit 2. The switching circuit 2 is driven in synchronism with a field synchronizing signal to divide an input image signal into first or odd field signals and second or even field signals. These odd and even field signals are supplied to right-eye and left-eye image displaying circuits 1R and 1L alternately.
The right-eye image displaying circuit 1R comprises amplification and separation circuit 3R for amplifying the odd field image signal and extracting a synchronizing signal therefrom, color demodulating circuit 4R, liquid crystal display module (LCD) 5R, horizontal and vertical timing circuit 6R, back-light 7R and back-light driving circuit 8R. The odd field image signal supplied from the switching circuit 2 is first amplified by the amplifying and synchronizing signal separating circuit 3R and then a synchronizing signal is extracted from the odd field image signal. The image signal is then supplied to the color demodulating circuit 4R to produce red, green and blue color signals R, G, B. These color signals are supplied to the LCD 5R. The synchronizing signal is supplied to the horizontal and vertical timing circuit 6R, and a scanning signal and a timing signal are generated. These signals are supplied to the LCD 5R to display the right-eye image thereon. The back-light 7R is driven by the back-light driving circuit 8R to illuminate the LCD 5R from its back.
Similarly, the left-eye image displaying circuit 1L comprises image amplifying and synchronizing signal separating circuit 3L, color demodulating circuit 4L, liquid crystal module (LCD) 5L, horizontal and vertical timing circuit 6L, back light 7L and back light driving circuit 8L, and the left-eye image is displayed on the LCD 5L.
Since the head-mounted type image display apparatus has to be small in size and light in weight, the image display device is formed by a liquid crystal display (LCD). When a DC voltage is applied to the LCD of TFT (Thin Film Transistor) type for a relatively long time, there is produced an electrical and chemical change within the LCD, so that characteristics of LCD might be deteriorated to a large extent. Therefore, in general, the LCD is driven by the image signal whose polarity is opposite to that of preceding fields by reversing the polarity of the image signal for every other field. Particularly, in case of displaying an interlaced image signal such as an NTSC signal on a small liquid crystal display having a size of 400.times.250 pixels, the same pixel is used for both the odd and even fields and the polarity of the image signal supplied to respective pixels is changed for successive fields. In this manner, the liquid crystal display is driven in an alternating current mode.
However, the LCD has such characteristics that an intensity of a displayed image is changed or fluctuated in accordance with a polarity of the image signal owing to the reason that a waveform of a voltage applied to the LCD is not symmetrical although the polarity of the image signal is reversed. Therefore, when the polarity of the image signal of successive fields is made opposite to that of preceding fields by reversing the polarity of the image signal for every other field, the intensity of the displayed image is changed in a rhythm of the field to produce a flicker.
In order to mitigate the above mentioned problem, there has been proposed to average the flicker by changing the polarity of the image signal for successive lines as shown in FIG. 2. By this measure, the flicker becomes imperceptible. If this solution is applied to the known image display apparatus shown in FIG. 1 and the polarity of the image signal is reversed for every other line, the image signal of an odd field is supplied to the right-eye display device 1R and the image signal of an even field is supplied to the left-eye image displaying circuit 1L. In this manner, the odd field images are displayed on LCD 5R and the even field images are displayed on LCD 5L. In this case, even if the polarity of the image signal is made opposite to that of preceding lines, the image signal having the same polarity is always supplied to pixels on the same line of LCDs 5R and 5L. Therefore, in each of the odd and even fields, the intensity of odd lines and that of the even lines are different from each other and respective lines of LCDs 5R and 5L are always driven by the image signal having the same polarity, and thus the life-time of LCDs 5R and 5L might be shortened.